John Kerry said a thorough review was being carried outSecretary of State John Kerry has said that in some cases, US spying has gone too far. Mr Kerry is the most senior Obama administration official to have commented directly on an issue that has upset America's European allies. He said he will work with the president to prevent further inappropriate acts by the National Security Agency.
But Mr Kerry also defended the need for increased surveillance, saying it had thwarted terrorist attacks.
"We have actually prevented airplanes from going down, buildings from being blown up, and people from being assassinated because we've been able to learn ahead of time of the plans," Mr Kerry told a conference in London via video link.
"I assure you, innocent people are not being abused in this process, but there's an effort to try to gather information. And yes, in some cases, it has reached too far inappropriately.
"And the president, our president, is determined to try to clarify and make clear for people, and is now doing a thorough review in order that nobody will have the sense of abuse," he said.
Claims about the extent of US surveillance of targets such as European leaders have strained Washington's diplomatic relations with some of its key allies. Last week it was alleged that German Chancellor Angela Merkel's mobile phone had been tapped for up to 10 years.
More recently there have been claims that the NSA hacked links connecting data centres operated by Google and Yahoo. The revelations stem from documents leaked by ex-US intelligence contractor Edward Snowden, who has been granted temporary asylum in Russia and is wanted in the US in connection with the unauthorized disclosures.
Where should a country draw the line vis-a'-vis spying? The security of any country depends largely on it's intelligence. And if they deny it, they are lying. In the wake of 9/11 it is understandable that America stepped up their program of espionage. A few hours warning or even less might have averted that tragedy.